Generally, PCM sound signals received via a satellite exits in the unit of frame and in a period of 1 msec., as shown in FIG. 1(A). Each frame of such PCM sound signals consists of 2048 bits, where previous 16 bits are synchronizing signals of "135E" Hex, and next 16 bits are control bits for controlling the decoding of sound data, while the remaining 2016 bits are range bits such as a plurality of sound bits, extra data, error correcting codes and the like.
A first bit of the above mentioned control bits indicates in A or B mode depending on whether its corresponding bit is 1 or 0. In the case of the A mode, 4-channel data having 32 KHz sampling frequency fs is decoded and an operation for expanding bits from 10 to 14 is carried out. On the other hand, in the case of the B mode, 2-channel data having 48 KHz sampling frequency fs is decoded and an operation for protecting an upper bits is carried out. Therefore, the reading of the A or B mode has to be precisely carried out for the control bits.
Second and third bits represent sound modes in a television indicating a whether a received sound mode is expressed in mode stereo mono a mode, a native language mode or a foreign language mode and the like as shown in table 1.
TABLE 1 ______________________________________ 0 1 ______________________________________ 0 Stereo Mono 1 Native/foreign Non-definition ______________________________________
Fourth and fifth bits represent an extra sound mode and also indicate a whether a received sound mode is expressed in mode stereo or a mono, mode a native language mode or a foreign language mode and independence as shown in table 2.
TABLE 2 ______________________________________ 0 1 ______________________________________ 0 Stereo Mono 1 Native/foreign Non-definition ______________________________________
Then, Sixth to fifteenth bits are not specifically defined, and a sixteenth bit indicates as to whether received sound data is to be muted at an end or not. Muting is carried out when the sixteenth indicates a logic state of bit "1".
It is well known that the sound data can be correctly decoded only when the control bits of the above described are correctly detected. However, the conventional PCM decoder which is constructed as shown in FIG. 2 can detect only the control bits simply in the unit of a frame. If a synchronizing signal indicating the start of one frame is detected by a synchronizing signal detecting section 7a after receipt of the PCM sound signals, a descrambler 7c controlled by a clock generator 7b disperses the format of the input PCM sound signals.
The dispersed data is supplied to a deinterleaving section 7h, where error dispersing operation is carried out for preventing simultaneous occurrence of errors. The deinterleaved signals are subjected to BCH (BASE-Chandhuri-Hocquenghem) codings by a certain sample block in a BCH correcting section 7i so that a single error is corrected and double errors remains uncorrected. The BCH-coded signals are supplied to a range bit extending section 7j, where condensed bits (condensed when transmitting) are extended.
An interpolating section 7k receives the extender range bit and the double errors found during the BCH coding to carry out a proper interpolating operation, and thereafter, the resultant data is supplied to an output selecting section 7l. The output selecting section 7l carries out the 32 or 48 KHz sampling depending on the selected mode of either A or B mode.
Meanwhile, the detection of the control bits has to be preceded to control the output selecting section 7l. Therefore, the control bit detecting section 7d detects the control bits in the unit of a frame among the PCM sound signals which are dispersed by the descrambler 7c. Then, the detected result is supplied to the output selecting section 7l and simultaneously supplied to a display control section so that the detected result of the control bits can be displayed on a front panel displayer (not shown).
However, the control bit reading method described above has the problem that the error occurrences in the control bits of the received sound signals could not be precisely detected.